Recently, as a cationic surfactant to be used in a fabric softener, in consideration of biodegradability when residues in drainage after a softening treatment are released in nature such as rivers, a cationic surfactant obtained by reacting triethanolamine, methyldiethanolamine, or the like with a long-chain fatty acid or a fatty acid methyl ester, thereby synthesizing an alkanolamine ester as an intermediate, and then, quaternizing the alkanolamine ester with a dialkyl sulfate such as dimethyl sulfate or diethyl sulfate is favorably utilized.
However, such a cationic surfactant having an ester group has a problem that an offensive odor is generated due to a sulfur (S)-containing compound such as methanethiol or dimethyl disulfide produced as a by-product derived from a quaternizing agent used during the production, deterioration of smell or hue due to long-term preservation during storage adversely affects the quality of a commercial product, or the like. With respect to such a problem, JP-A-8-507756 discloses a method in which a peroxide and an alkali metal borohydride are added to the alkanolamine ester before quaternization, and also JP-A-4-506804 discloses a method in which air is brought into contact with the alkanolamine ester before quaternization.
The methods disclosed in the prior art such as JP-A-8-507756 and JP-A-4-506804 are not sufficiently effective in improving the quality with respect to smell and hue after production and during storage. The present invention provides a method for producing a cationic surfactant which is useful as a softening base, has favorable biodegradability, is prevented from smelling and coloring, and has excellent storage stability, and also provides a cationic surfactant produced by the method.
According to the present invention, a method for producing a cationic surfactant including the following step 1, step 2, step 3, and step 4 is provided:
step 1: a step of obtaining an alkanolamine ester by reacting an alkanolamine with a fatty acid or a fatty acid alkyl ester without using hypophosphoric acid or a salt thereof;
step 2: a step of obtaining a cationic surfactant by quaternizing the alkanolamine ester obtained in the step 1 with a dialkyl sulfate;
step 3: a step of performing an oxidation treatment of the cationic surfactant obtained in the step 2; and
step 4: a step of performing a reduction treatment of the cationic surfactant subjected to the oxidation treatment obtained in the step 3.
According to the present invention, a high quality cationic surfactant, which is prevented from smelling and coloring and has favorable storage stability, can be obtained by performing an esterification reaction or a transesterification reaction of an alkanolamine thereby obtaining an alkanolamine ester, quaternizing the alkanolamine ester, and then, performing an oxidation treatment and a reduction treatment in this order. The obtained cationic surfactant is useful as a softening base to be used in a fabric softener, and also has favorable biodegradability.